1. Technical Field
The present invention relates in general to communication between mobile users of and in a computer network and in particular to a method and an apparatus for establishing a way of providing secure identification of a mobile user in a communication network.
2. Description of the Related Art
In today's communication networks, user mobility is rapidly becoming an important and popular feature, particularly in wireless or cellular networks. While useful and desirable, this increased user mobility leads to a number of important security-related issues and concerns. One issue is the approval or acceptance of the user; another issue is the tracking a mobile user's movements and current whereabouts.
A typical situation arising in mobile environments is when an entity, i.e. a user or a device, registered in a particular home domain, appears in a different, i.e. foreign domain. Presumably, this user's goal is to obtain certain services while in the foreign domain. Since this user is not known in the foreign domain, he/she must be authenticated and his/her "solvency" or good standing must be confirmed to the authority of the foreign domain. Within the following specification, this process is denominated "authentication", as usual in the art. Of course, the only entity able to comment on the user's identity and current standing is the authority in his/her home domain. There are several known solutions to this problem in the recent literature, some of them are addressed below. However, authentication is not the issue that the present invention addresses.
Of concern here is another security-related issue arising as a result of user mobility. It is the confidentiality of the user's identity and his/her movements. Ideally, only the user's home domain authority should be informed as to the mobile user's itinerary and current whereabouts. In the following, this process of establishing the identity of a mobile user, i.e. of determining who is trying to obtain a service from a particular domain actually, is denominated "identification".
Ideally, no entity other than the user himself/herself and a responsible authority in the user's home domain, i.e. the subnetwork or partition of the network within which the user typically works, should know the real identity and/or the current location of the mobile user. Current environments supporting user mobility either do not address the problem at all or base their solutions on hardware capabilities of the user's personal device.
Generally, one may say that the known solutions for this problem offered by current state-of-the-art mobile/cellular architectures are either inadequate or too specific to assure a secure identification in secrecy, as detailed below.
One of the presently available solutions is reported by M. Rahnema in (1). In this so-called GSM system, the mobile user is routinely assigned a temporary identity (TMSI, in GSM parlance) when he/she appears in a foreign domain. However, a TMSI is only assigned after the initial authentication of the mobile user in the foreign domain; in the process carried out by the latter, the user's real identity (IMSI, in GSM parlance) is communicated in the clear and can thus be recognized and misused by an intruder.
Another solution is described in a specification (2) on a "Cellular Digital Packet Data" (CDPD) system. The approach taken by the CDPD system is more secure than in the above GSM solution. In the CDPD system, before a mobile user communicates his/her identity, he/she engages in a Diffie-Hellman key exchange protocol with the local, i.e. foreign, domain authority. This protocol is described by W. Diffie and M. Hellman in (3). As a result, both parties come to share a secret key. Enciphered under this key, the mobile user subsequently transmits his/her identity to the foreign domain authority.
While more secure than GSM, this approach has two major drawbacks. First, it allows the local, i.e. foreign, domain authority to discover the real identity of the mobile user. In the context of CDPD, this is not a problem in and of itself. However, ideally, the identity of the mobile user should not be revealed to the local domain authority. It is sufficient for establishing his/her identity and current standing if it is corroborated or endorsed by the home domain authority. The second problem is due to the nature of the Diffie-Hellman key exchange protocol. Its purpose is to establish a secret key on-the-fly. This allows an intruder to masquerade as the local domain authority and thus to engage in the key exchange protocol with the mobile user and obtain a shared key. When the mobile user then transmits its real identity enciphered with this same key, an intruder will simply decipher the transmission.
Other approaches are given by R. Molva et al in (4) and by M. Beller et al in (5). One side aspect, relating to key distribution, is described in Applicant's PCT Application PCT/EP93/01989 (6), another side aspect, relating to password or key change, is addressed in Applicant's PCT Application PCT/EP93/02540 (7).
In summary, there are essentially three issues underlying the problem of mobile user identity and movement confidentiality.
The central issue in maintaining a secret identity is to prevent anyone from discovering a correspondence between a mobile user and a user registered in a particular home domain, in other words, the central issue is to keep the user's identity confidential. The easiest, rather intuitive solution is to assign a travelling alias to every mobile user or device when away from the home domain. As addressed below, this alias can be fixed or ever-changing. Consequently, a main object of the invention is to devise a method and a system that is adapted to and permits the use of such aliases.
The second important issue is to keep foreign domains "in the dark". If it is not imperative for a foreign domain to know the real user's identity, an alias should suffice. In most cases such an alias must still be corroborated by the home domain authority. Consequently, another object of the invention is to design a method and a system which enables the information to flow through the network without revealing the identity of the user to the foreign domain. (Whether or not aliases are used, there may be reasons why the foreign domain authority still demands to know the real identity of the user. In this case, the home domain authority may communicate the user's identity in secret, assuming, of course, that the two authorities have a pre-established means for secure communication. However, even in this case, the foreign domain originally does not know the user's identity.)
The third issue of particular concern is to prevent identity tracking or correlation. Even if a mobile user adopts a travelling alias, his/her movements can still be tracked by a hostile intruder. This is especially possible if the alias is fairly static, e.g. fixed for a given trip of a user or permanently allocated to said user. An alias of this latter type is similar to a long-term password; once cracked, the identity and the movements of the user can be compromised on a long-term basis. Consequently, a further object of the invention is to prevent the tracking by devising a system geared and adapted to use frequently changing aliases without inhibiting the information flow.
References
(1) Rahnema: "Overview of the GSM System and Protocol Architecture", IEEE Communications Magazine, April 1993, Vol. 31, No. 4; pp. 92-101. PA1 (2) "Cellular Digital Packet Data (CDPD) System Specification", Release 1.0, Jul. 19 1993, CDPD Industry Input Coordinator, Costa Mesa, Calif., USA. PA1 (3) W. Diffie and M. Hellman: "New Directions in Cryptography", IEEE Transactions on Information Theory", November 1976, Vol. 22, No. 6, pp. 644-654. PA1 (4) R. Molva, D. Samfat, G. Tsudik: "Authentication of Mobile Users", IEEE Network, Special Issue on Mobile Communications, Spring 1994, pp. 25-35. PA1 (5) M. Beller, L. Chang, Y. Yacobi: "Privacy and Authentication on a Portable Communications System", IEEE JSAC, Special Issue on Wireless Personal Communications, August 1993, Vol. 11, No. 6, pp. 821-829. PA1 (6) Patent Application PCT/EP93/01989, entitled "Method and Apparatus for Providing Secure Key Distribution in a Communication System", by IBM Corporation and P. Janson, G. Tsudik. PA1 (7) Patent Application PCT/EP93/02540, entitled "Method and System for Changing an Authorization Password or Key in a Distributed Communication System", by IBM Corporation and R. Hauser, P. Janson, R. Molva, G. Tsudik, E. van Herreweghen. PA1 (8) US National Bureau of Standards: "Federal Information Processing Standards", Publication 46, 1977. PA1 (9) R. Rivest: "The MD5 Message Digest Algorithm", Internet RFC 1321, Internet Activities Board, April 1992. PA1 (10) R. Molva and G. Tsudik: "Authentication Method with Impersonal Token Cards", 1993 IEEE Symposium on Research in Security and Privacy, May 1993, Proceedings published by IEEE Computer Society Press, Los Alamitos, Calif., USA. PA1 (11) Security Dynamics Technologies, Inc., Cambridge, Mass., USA: "The ACE System Access Control Encryption", Product Information, 1992.